All ungulates are mammals (Class Mammalia) - a group of warm-blooded
(endothermic) animals with backbones. Taxonomically, mammals are animals
(Kingdom Animalia) and belong to the Phylum Chordata, Subphylum Vertebrata.
Mammals can be distinguished from other animals by the presence of milk-producing
skin glands - these mammary glands, for which the Class Mammalia is named,
are found in every mammal and nowhere else in the Animal Kingdom. Many other
characters are used to diagnose mammals, including the presence of hair,
three middle ear bones, and a four-chambered heart.

Within the mammals, there are two extant subclasses: the Prototheria, containing
only three modern species of egg-laying monotremes (Order Monotremata); and
the Theria, or viviparous mammals (those which give birth to live young).
The Theria are subdivided further into the marsupials (infraclass Metatheria),
and the eutherians or placental mammals (Eutheria). Ungulates - and most
of the other 4,500+ mammal species - are eutherians, mammals which nourish
their young inside the mother's body using a chorioallantoic placenta (in
eutherians, the placenta acts as an endocrine organ during pregnancy). The
extended time which young can develop inside their mothers gives infant
eutherians a head-start in life, and is responsible for the ability of many
ungulates to run only hours after birth.

The two ungulate orders (Perissodactyla
and Cetartiodactyla) do not isolate from
the rest of the mammals together, and thus they do not form a distinct taxonomic
group. As we saw in What is an Ungulate?,
our understanding of these relationships is rather recent, and is still making
its way through the scientific literature. Because the true ungulates no
longer stand alone, but are instead nested within the infraclass Eutheria
(and the superorder Laurasiatheria), each order must be examined individually.

The easiest way to distinguish the two types of ungulates is to look at their
feet (and hooves): Perissodactyls (generally) have an odd number of toes
on each foot, while "artiodactyls" (the ungulates within Cetartiodactyla)
usually possess an even number of toes on each foot. Despite the even-odd
differentiation, there are exceptions to the rule: for instance, both tapirs
(Perissodactyla:
Tapiridae) and some peccaries
(Cetartiodactyla:
Tayassuidae) have four toes
(and hooves) on their front feet, and three on the rear. How is this difference
accounted for? It is not officially the NUMBER of toes, but their ARRANGEMENT,
that differentiates the two orders. Perissodactyls have a mesaxonic foot
structure, where the plane of symmetry of the foot passes through the third
digit (this digit also bears most of the animal's weight). In artiodactyls,
the third AND fourth digit are the principal weight bearers (i.e., the axis
of the foot passes BETWEEN the two toes), with the result that their foot
structure is paraxonic.